Golf balls are typically constructed of a single or multilayer core that is tightly surrounded by a single or multilayer cover. It is typical for a golf ball core to be of solid construction or wound construction. A solid core commonly comprises polybutadiene, and a wound core typically comprises rubber threads tightly wound around a solid or liquid center. The methods for forming these cores are well known in the art. Traditionally, golf ball covers are made of polymeric materials. For instance, covers have been made of balata rubber, which may be natural balata, synthetic balata or a blend of natural and synthetic balata, or ionomers such as those sold under the trademark SURLYN.RTM..
The Rules of Golf as approved by the United States Golf Association (USGA), includes the following rules that relate to golf ball construction:
a. Weight
The weight of the ball shall not be greater than 1.620 ounces avoirdupois (45.92 gm).
b. Size
The diameter of the ball shall be not less than 1.680 inches (42.67 mm). This specification will be satisfied if, under its own weight, a ball falls through a 1.680 inches diameter ring gauge in fewer than 25 out of 100 randomly selected positions, the test being carried out at a temperature of 23+/-1.degree. C.
c. Spherical Symmetry
The ball must not be designed, manufactured or intentionally modified to have properties which differ from those of a spherically symmetrical ball.
d. Initial Velocity
The velocity of the ball shall not be greater than 250 feet (76.2 m) per second when measured on apparatus approved by the United States Golf Association. A maximum tolerance of 2% will be allowed. The temperature of the ball when tested will be 23+/-1.degree. C.
e. Overall Distance Standard (ODS)
A brand of golf ball, when tested on apparatus approved by the USGA on the outdoor range at the USGA Headquarters under the conditions set forth in the Overall Distance Standard for golf balls on file with the USGA, shall not cover an average distance in carry and roll exceeding 280 yards (256 m) plus a tolerance of 6%.
The Initial Velocity rule test is well known. The test is conducted by conditioning a ball for a minimum of 3 hrs at 23.+-.1.degree. C. The room in which the test is to be conducted is conditioned to 23.+-.2.degree. C. The ball is then struck by a striking mass of approximately 250 lbs at a striker velocity of 143.8 ft/sec.
The Overall Distance Standard test for golf balls is also well known and is conducted by striking the golf ball with a golf club having a lie of 55.+-.2, a D2.+-.1 swing weight, 11.+-.10 loft angle, 13.3.+-.1 oz. overall weight, 4.6.+-.0.3 cycle per second vibrational frequency, 162.+-.30 oz. in. moment of inertia, 43.5.+-.0.2 inches club length, a laminated head construction, a cycolac insert, a stiff steel shaft, and an all-weather rubber grip. The club head velocity during the ODS test is about 160.+-.0.5 ft/sec measured at the hosel of the club over the last 4 inches of travel prior to impact. The distance traveled is adjusted for zero wind and an ambient temperature of 75.degree. F. The setup of the specially designed mechanical-golfer apparatus of the ODS test generally produces an initial ball velocity of 225-235 fps, a launch angle of 8.degree.-11.degree., and a spin rate of 2300 to 3000 rpm using a known calibration ball.
The flight of a golf ball is determined by many factors, but only three factors that are typically controlled by the golfer. By impacting the ball with a golf club, the golfer typically controls the speed of the golf ball, the launch angle, and the spin rate. The launch angle sets the initial trajectory of the golf balls flight. The speed and spin of the ball give the ball lift which will define the balls overall flight path along with the weight and drag of the golf ball. Where the ball stops after being struck by a golf club also depends greatly on the weather and the landing surface the ball contacts.
Many golfers have what is termed a "low swing-speed." This means that the club head speed at impact is relatively slow when compared to that of a professional golfer. Typically, an average professional can drive a golf ball at a speed of approximately 235 fps (160 mph). A person having a low swing speed typically drives the ball at a speed of less than 176 fps (120 mph). Most golfers today have swing speeds that produce drives of less than 210 yards. A person with a low swing speed generates a low ball speed. His or her ball does not fly very far due to the lack of speed and lift.
Low weight golf balls have been suggested in the past, such as the Cayman Golf Company's SPECTRA.TM., the Ram LASER LIGHT.RTM., the Maxfli MD-80 and MD-90, and the Pinnacle EQUALIZER.RTM., which weighs 1.55 oz. and has a hard core and a standard cover with 392 dimples. Low weight golf balls such as these have been made to increase the lift to weight ratio of the golf ball, increasing the effects of the lift on ball trajectory, and also to produce a greater initial velocity upon impact than a heavier ball. It is generally known that low weight golf balls slow down faster than normal weight golf balls due to drag, an effect which is magnified at higher speeds. As a result, low weight balls have been designed to increase the lift to weight ratio, but not particularly to decrease the effect of drag.
Attempts have been made in the past to minimize drag, but these attempts have been focused only within a narrow window optimized for higher swing-speed ball flight. Conventional dimple patterns optimize lift to create the best trajectory for maximizing distance. These conventional dimple designs, however, are aerodynamically optimized for higher swing speeds than low swing-speed players can achieve. Generally, as the lift of a dimple pattern increases, drag also increases.
The advantages of a high spin, high lift golf ball are minimized at the low air speeds achieved by a ball hit by low swing-speed player. Since lift force increases with the square of the ball speed, ball speed has a greater effect than ball spin in creating lift, which produces a higher trajectory. When a player strikes a ball, a portion of the energy from the club head is transferred to the ball as ball speed, and another portion of the energy is transferred to the ball as ball spin. Players with low club swing speed necessarily will have less energy available to transfer to both ball speed and ball spin. When club speed becomes very low, the resulting ball speed can be low enough that the effect of ball spin does not significantly increase lift force. When the lift force exceeds the weight of the ball, the flight of the ball is described as "aerodynamic." When the lift is less than the weight of the ball, the ball flight is described as "ballistic." Aerodynamic flight can generally be optimized by balancing the relationship between ball speed, ball spin, and the aerodynamic effect of the dimples. Ballistic flight can generally be optimized by increasing the ball velocity, as a lift modifying effect of the dimples is minimized. When compared to a ball hit by a high swing-speed player, a similar ball that is hit by a low swing-speed player travels along a more ballistic trajectory than the aerodynamic trajectory achieved with a higher energy of a high swing-speed player.
The dimples on a golf ball play an important part in reducing drag and affecting lift. Drag is the air resistance that acts on the golf ball in the opposite direction from the ball's flight direction. As the spinning ball travels through the air, the air surrounding the ball has different velocities and, thus, different pressures. The air exerts maximum pressure at the stagnation point on the front of the ball. The air then flows over the sides of the ball and has increased velocity and reduced pressure. At some point it separates from the surface of the ball, leaving a large turbulent flow area behind the ball called the wake, which has low pressure. The difference between the high pressure in front of the ball and the low pressure behind the ball slows the ball. This is the primary source of drag for a golf ball.
The dimples on the ball create a turbulent boundary layer around the ball, i.e., the air in a thin layer adjacent to the ball flows in a turbulent manner. The turbulence energizes the boundary layer and helps it remain attached further around the ball to reduce the area of the wake. This greatly increases the average pressure behind the ball and substantially reduces the drag.
Lift is the upward force on the ball that is created from a difference in pressure between the top of the ball and the bottom of the ball. The difference in pressure is created by a warpage in the air flow resulting from the ball's back spin. Due to the back spin, the top of the ball moves with the air flow, which delays the separation to a point further aft. Conversely, the bottom of the ball moves against the air flow, moving the separation point forward. This asymmetrical separation creates an arch in the flow pattern, requiring the air over the top of the ball to move faster, and thus have lower pressure than the air underneath the ball.
Most golf ball manufacturers research dimple patterns in order to increase the distance traveled by a golf ball. A high degree of dimple coverage is beneficial to flight distance, but dimple coverage gained by filling spaces between normally sized dimples with tiny dimples is not very effective because tiny dimples are not good turbulence generators. Most balls today have many large spaces between dimples, or have filled these spaces with very small dimples that do not create enough turbulence at average golf ball velocities.
Low drag, low lift dimples are generally smaller than in common golf balls and cover a large area of the surface of the golf ball. Such dimples are taught, for example, in U.S. Pat. No. 4,560,168. This patent teaches a variety of dimple patterns for golf balls. The dimple pattern is obtained by dividing the spherical surface of the golf ball into twenty spherical triangles corresponding to the faces of a regular icosahedron. Each of the twenty triangles is further divided into four smaller triangles: one central triangle and three apical triangles at the three apexes of the larger triangle. This is done by connecting the midpoints of the sides or edges of the larger triangle by great circle paths. The dimples are arranged in each central triangle and each apical triangle such so that no dimples intersect the edges of the central triangle. The patent states that the size, numbering, and configuration of the dimples should be selected to optimize aerodynamic performance and minimize bald patches.
A golf ball with an improved ballistic, as opposed to aerodynamic, trajectory is needed to improve play for low swing-speed golfers.